Graphene is an allotrope of carbon, which contains a single atomic sp2 carbon arranged in honeycomb fashion making a two-dimensional sheet. This material exhibits various unusual properties such high electron mobility (2×105 cm2 V−1 s−1) at room temperature, thermal conductivity (5×105 Wm−1K−1) and high Young's modulus (˜1 TPa) [Science, 306, 666, 2004; Nature, 438, 197, 2005; Phys. Rev. Lett., 100, 016602, 2008; Science, 321, 385, 2008]. Besides, coating of graphene over the surface of metal improves the corrosion resistance of metal [ACS Nano, 6, 1102, 2012]. Moreover, the properties of graphene have been tailored by substituting heteroatom like oxygen on the graphene sheet. Such graphene with the heteroatom has been coined as a “reduced graphene oxide” in the literature (Angew Chem. Int. Ed. 49, 9336, 2010). Several methods have been reported in the literature for the deposition of graphene on metal surface, which includes chemical vapor deposition (CVD) method. However, CVD process is expensive and is incapable of a large scale production.
Several methodologies have been patented on coating of graphene or reduced graphene oxide on the surface of metals sheets. US 20130143067 A1 discusses about coating of graphene on metal plates such as steel, aluminum, copper and its wire by using chemical vapor deposition and spray technique. The spray technique uses polymer mixed reduced graphene oxide, which is to be a composite instead of pure reduced graphene oxide. Therefore, the metal sheet coated with the composite will have different properties than the metal coated with the pure reduced graphene oxide. Reference may also be made to a patent US 20090110627 A1, wherein the synthesis of graphene sheet is made from different carbon sources, which includes carbon monoxide, alcohols and polycyclic aromatic compounds. US 20110104442 A1, also describes deposition of graphene by CVD method directly on substrates using carbon sources other than graphite. US 20100021708 A1 discloses the deposition of graphene on a SiO2/Si substrate. However, the deposition process is very slow because it involves several coating and chemical etching steps.
Sun et al. reported the growth of graphene from Poly(methyl methacrylate (PMMA) and sucrose on Cu substrate [Nature 468, 549, 2010]. WO 2012064102 A3 disclosed the coating of graphene on steel using acetylene gas as a carbon source. Reference may also be made to a patent US13704054 A1, which claimed coating of graphene on steel surface for corrosion resistance. This method comprises the synthesis of reduced graphene oxide from graphite powder by oxidation-reduction process followed by mixing with polyimide. Subsequently, the reduced graphene oxide-polyimide composite was coated on the steel surface. However, process is time consuming and polyimide has poor adherence to steel surface. In light of the above discussed references, it is evident that graphene coating has been disclosed and developed using various carbon sources and CVD technique however, the large scale production remain a challenge.
On the other hand, coating of graphene by solution method is well established [Int. J. Chem. Eng. App., 3, 453, 2012] and is capable of a large scale production. The solution method follows the following procedure; (a) graphite powder needs to be oxidized to minimize the vander Walls forces between the two graphitic layers, (b) the prepared graphite oxide is dispersed in water by sonication, (c) the dispersed graphite oxide solution is reduced using the reducing agents such as hydrazine, HI acid, NaBH4, citric acid etc. (d) the reduced product is dispersed in a suitable organic solvent and is used for coating. But, this method involves several disadvantages that limit the process to be used at industrial scale for the following reason; the conversion of graphite powder to graphite oxide involves several steps which is time consuming and uses some harmful acids like HNO3 and H2SO4. Further, reducing agents are also toxic, handling them in large scale is a problem and the reduced product has a very low dispersity in organic solvent and hence, requires sonication for a long time. Also, the adherence of the graphene sheets with the metal surface is very low. Besides, the graphite used for the carbon source is expensive.
Some reports show that natural carbon source like flower petal can be used to produce graphene in a large scale at low cost [Carbon 50, 4123, 2012]. However, graphene produced from natural sources finds difficulty to integrate on the metal/alloy surfaces, therefore, finds limited application in the area of graphene coated metal sheets.
Hence, there is need of an invention that can synthesize graphene at large scale using available carbon sources. Further, process should be cost effective and commercially viable.